High Nitrate spike

Hello this is my first post so sorry if it is in the wrong section, but I desperately need help.
Ok so I can give you all the wonderful information I have a 240 gallon with two 4 inch oscars, a 10 inch arrowana, a 3 inch peacock chiclid, and a 3 inch red tail cat.
I also have a 130 gallon planted aquarium with 10 mollies, a 1 inch florida gar, and a 2 inch nigerian bichir.
I do water changes every Saturday on these tanks about 25% and my readings have always been Amonnia 0 nitrite 0 nitrate about 5.
This Saturday I did the water change and then on Sunday we noticed fish acting lathargic in the 240 gallon so I checked the water and everything is still 0 except nitrate which has topped out on my color guide to 80 on both tanks.
I tested the tap water and it is reading 5 I did a 50% water change on both tanks I checked the nitrate again today and it is back to 80 on both tanks.
Does anybody have any idea why this would be happening in both tanks when the tap is reading 5?
If the kit is bad wouldn't it read high from the tap as well?
Here is a photo of my test

Thank you very much for the reply
I do shake the kit for at least 30 seconds before testing but its probably almost a year old and almost empty.
Should I clean all that at once wouldn't it mess up my cycle to clean all the filters?
also I have sand substrate so all the food usually stays on top and I suck it out weekly.
I have done three water changes in the last three days but it is still reading high so im not sure if water changes are helping but ill try another.
The mian thing im worried about is my tiny baby gar I dont want him shocked from all this and end up dead on me=(

Here all I have is petsmart that's close and they use strips that they have sitting out and open so I usually don't trust them to much I guess I'll pick up a new kit and see if it changes.
I do stir the sand weekly and move the driftwood around I also try to get around the plants without uprooting them but that does get hard to do